Field of the Invention
The invention relates to a liquid crystal alignment agent, a liquid crystal alignment film, and a liquid crystal display (LCD) element. More particularly, the invention relates to a liquid crystal alignment agent allowing formation of an LCD element having good reliability and less residual image, a liquid crystal alignment film formed by the liquid crystal alignment agent, and an LCD element having the liquid crystal alignment film.
Description of Related Art
The LCD is widely applied in, e.g., television and various monitors. LCD elements having the following types of liquid crystal cell are known: twisted nematic (TN)-type, super-twisted nematic (STN)-type, in-plane switching (IPS)-type, and fringe field switching (FFS)-type that changes the electrode structures of IPS-type, etc. and increases brightness by increasing the aperture ratio of the display part.
The following methods are known for aligning the liquid crystals of these liquid crystal cells: forming an organic film such as a liquid crystal alignment film on the surface of the substrate and using a cloth material such as rayon to rub the surface of the organic film in a certain direction, depositing silicon oxide on the surface of the substrate through oblique evaporation, using a Langmuir-Blodgett (LB) method to form a monomolecular film having long-chain alkyl groups, and so on. In particular, from the viewpoint of the substrate size, the uniformity of liquid crystal alignment, the treatment time, and the treatment cost, a rubbing treatment is most commonly used.
However, if a rubbing treatment is used to perform liquid crystal alignment, dust may adhere to the surface of the alignment film due to the dust or static electricity generated during the process, thus causing poor display. In particular, for a substrate having thin film transistors (TFTs), the generated static electricity causes damage to the circuit of TFTs, thus reducing the yield. Moreover, for the LCD element will become more and more delicate, with the densification of the pixels, the surface of the substrate becomes uneven, and therefore it is difficult to perform a uniform rubbing treatment.
As a result, to avoid such undesired situation, a photoalignment method, such as that described in JP 2005-037654 A, provides liquid crystal alignment capability by irradiating polarized or non-polarized radiation on a photosensitive film. The patent literature provides a liquid crystal alignment agent that has a repeating unit containing conjugated enone and has an imide structure. Thereby, static electricity and dust are not generated, and therefore uniform liquid crystal alignment can be achieved. Moreover, in comparison to the rubbing treatment, the method can precisely control the direction of liquid crystal alignment in any direction. Furthermore, by using, e.g., a photomask in the radiation irradiation, a plurality of regions having different directions of liquid crystal alignment can be formed on one substrate in any manner.
However, the LCD element obtained from the liquid crystal alignment film has the drawbacks of poor reliability and having residual image, and is not acceptable to the industry. Thus, how to provide a liquid crystal alignment agent capable of forming a liquid crystal alignment film having good reliability and less residual image, such that a better display quality can be achieved when the liquid crystal alignment film formed thereby is applied in an LCD element, is an issue that need to be addressed urgently.